In a conventional optical disk used in a CAV (constant angular velocity) mode, a servo byte interval is cyclically provided at a predetermined position of each track. A clock pit for generating a reference clock and a wobbled pit for effecting tracking are formed in this servo byte interval. A reference clock (channel clock) is generated in accordance with the clock pit and information is recorded digitally by a pit whose length is an integral multiple of the period of this reference clock. Further, in the system such as a CD (compact disc) that is used in a CLV (constant linear velocity) mode, although there exists no clock pit, a length of the recorded pit and the pit interval are selected so as to become a length (9 kinds of lengths ranging from about 0.9 to 3.3 .mu.m in the case of CD) of an integral multiple of the reference clock (channel clock). Then, a clock is reproduced by using the clock pit and recorded information is sampled at the unit of bits.
In a video disc which is the same optical disk, a video signal is recorded and reproduced by a difference of a pit considerably smaller than that of the CD.
This fact will be described with reference to a signal recorded on a portion of a radius 55 mm in the CAV mode. In the video disk, a brightest portion in a video signal is recorded as a signal of 9.3 MHz and a darkest portion is recorded as a signal of 7.6 MHz, and these signals are equivalent to 1.075 .mu.m and 1.316 .mu.m on a disk having a radius of 55 mm, respectively. It is a well-known fact that, if the disk thus recorded is reproduced, then a very beautiful picture is reproduced. Considering that the change of brightness of 128 gradations can be expressed in this picture, then this means that the period of the pit is recorded on the disk in the gradation of 128 or more and then reproduced therefrom. That is, the change of fine pit length and pit interval of EQU (1.316 .mu.m-1.075 .mu.m)+128=0.002 .mu.m
is reflected on the video signal.
With respect to the change of the pit length, the reason that the minimum unit of the change of the pit length must be increased to be 0.3 .mu.m regardless of the fact that the very small change can be recorded is mainly assumed to be such that a recording and reproducing method is not optimum.
The present applicant has previously proposed, as Japanese patent application No. 3-167585, a method in which digital information is recorded by shifting the position of a front or rear edge of an information pit in a step-wise fashion from a predetermined reference position in response to recording information. According to this recording and reproducing method, since the change of the position of the pit length and pit edge can be detected with a very high accuracy, it becomes possible to record information by a very small change that is considered to be impossible. As a result, the recording with higher density than ever can be realized.
FIG. 33 shows a principle of recording information by shifting in a step-wise fashion the position of the edge as the present applicant has previously proposed. As shown in the figure, a recording signal (FIG. 33B) that is PWM-modulated is generated in response to recording data. Then, a pit (FIG. 33A) corresponding to the length in zero-cross is formed. With this arrangement, the position of the edge of the pit is changed in a step-wise fashion from the position indicated by a reference clock (FIG. 33C). Data of 8 stages (3 bits) from 0 to 7 can be recorded per edge in response to the amount of such change.
FIG. 34 shows a principle of reproducing the signal thus recorded. A binary RF signal (FIG. 34B) is obtained by considerably amplifying an RF signal (FIG. 34A) reproduced from the information recording medium. Because the clock pits are formed on the disc in which information is recorded, a reference clock (FIG. 34C) is generated on the basis of the clock pits, and further a sawtooth wave signal (FIG. 34D) is generated in synchronism with this reference clock. Then, the position of the edge of the information pit is detected by detecting a timing at which the sawtooth wave signal and the binary RF signal cross each other.
However, in the previously-proposed arrangement, there occurs an intersymbol interference between the adjacent edges. Further, if the recording density is increased, there is then the problem that the accurate reproduction becomes impossible.
It is considered to use an equalizer in order to reduce the influence of the intersymbol interference. For example, the reproduced RF signal is spaced apart by a constant distance .DELTA. by using 3-tap equalizer and then sampled three times. Then, three values are processed by a linear calculation. An impulse response in this case is expressed EQU h(t)=.delta.(t)-.kappa.{.delta.(t+.DELTA.)+.delta.(t-.DELTA.)}
Therefore, its frequency response is expressed as follows: EQU H(f)=1-.kappa. cos(2.pi..DELTA.f)
By properly selecting .DELTA. and .kappa., the influence of the intersymbol interference can be reduced while the signal component of the high frequency region can be emphasized.
However, since there is performed the linear calculation, this processing cannot be perfectly applied to a nonlinear intersymbol interference. Also, if a value of a coupling coefficient .kappa. is increased in order to increase a degree of eliminating the intersymbol interference, a noise component of the high frequency region is emphasized, which presents a contrary effect.
Further, in the previously-proposed arrangement, when the position of the edge is detected, the sawtooth wave is generated and the timing at which the edge is produced is detected from the sawtooth wave. Therefore, the arrangement for reading this timing at which the edge is produced becomes complicated. There is then the problem that it becomes impossible to accurately detect such timing.
Furthermore, in the previously-proposed arrangement, influences such as an amplitude fluctuation peculiar to the optical disk and a fluctuation of the bias component are not taken into consideration at all. There is then the drawback that correct data cannot be read out due to these fluctuations.